Parafoil Kites

Most kites use thin sheet material for their sails. It is well known,
however, that this is not the most efficient lifting surface, and aircraft
wings invariably have a pear-drop cross section which causes the air passing
over the top to travel further than the air passing underneath, so producing
a reduction in air pressure above the wing with minimal turbulence.
Aircraft use rigid structures but these would be far too heavy
to fly as a kite in reasonable wind speeds.

Parafoil

The solution is the parafoil kite, invented in 1963 by Domina Jalbert of
Florida. An aerofoil shape is created as a structure inflated by
wind entering vents in the front.
(Jalbert's original
patent
is online.)
Normally, no spars are used.
Triangular flares sewn into the underside improve stability, to which the
multi-legged bridle is attached. Some models are flown with a drogue to help
stabilise them, even though this compromises the high efficiency of the
basic design.

Parafoils are extremly efficient and can be made in a large range of sizes
by altering the size and number of cells. In the larger sizes they are
capable of lifting enormous loads and need to be treated with considerable
respect. Here is a
good size one and here are
plans
for another (in French).

Flowform

As already mentioned, crosswinds and gusts can destabilise a parafoil, in
the worst case, causing it to deflate and fall to the ground. Pressure
differences from cell to cell cause subtle changes in the aerofoil shape and
this, together with the lack of any rigid structure, can cause variations in
the angle of attack across the kite, leading to instability. These problems
are addressed in a variant of the parafoil invented in
the late '60s by Steven Sutton, a Canadian parachutist. His design differs
in several subtle but important respects. The front vents are larger, and
vents are introduced in the rear of the centre cells and the vertical ribs
separating the cells, allowing a continuous flow of air through the kite.
Additionally, vents are made in the upper and lower surfaces. These
modifications are designed principally to equalise pressure, with the vents
in the upper and lower surfaces acting as inlet or exhaust vents according to
local conditions. Furthermore, the vents in the upper surface help to delay
stalling at high angles of attack, making the flowform easier to launch and
rock-steady in a wide range of winds.

Visually, the flowform differs little from a standard parafoil, the key
distinguishing feature being the vents in the rear of the central cells, which
appear like a bite out of the trailing edge. In some models, this is
very deep, giving the whole kite the appearance of a molar.
The vents in the upper and lower surfaces can also be seen if you look
carefully.

Here is a
picture of a flowform.
Outline
plans
are given by Carl Crowell, or a more detailed
plan
is available but only in French.
Or, you can find full constructional details in
Rowlands (But check the
errata
before you go too far.)

Inflatable Kites

Another variant of the parafoil is the inflatable kite,
in which a standard parafoil is sculptured into some "fun" shape. One
of the best known of these is "Martin's legs". Designed by Martin Lester,
this looks in flight like a lower torso, consisting of a parafoil with
two extended cells to form the legs, which kick entertainingly in the wind.
In the same vein, many other creations have taken to the sky (with
greater or lesser alacrity), limited only by the imagination and ingenuity
of the designer.
Here's a collection of Martin Lester's
body parts flying together. New Zealander Peter Lynn has
long been famed for his kites of many types, from kids' kites to world
record holders. Here's his
Megabite His
large
kites webpage contains pictures of his Trilobite, as well as the
MegaBite and Mega-Ray with effective areas of over 600 sq m! Some of
these are so large that on a sunny day, the solar heating effect warms
the air inside them so much that they hardly need any wind to fly.

A few years ago I ventured to ask on rec.kites how on earth you would
set about designing one of these monster kites. I got this brilliant
response
from Steve Lawrie. It starts out serious but ends up highly entertaining.

Plans by Bernhard Malle are available for an
octopus after Peter Lynn's.
Rowlands is a good source of
plans and general advice on parafoils, flowforms and inflatables.
Pelham and
Eden also have plans for parafoils.